If you’ve ever walked out of the optometrist’s office, prescription in hand, and been completely puzzled by the seemingly arcane markings on the page, you’re not alone.
While not purposely confusing, an eyeglass prescription uses some language and short forms that aren’t in common English usage. Here’s the key to the code:
OD and OS
Unless you majored in Latin at university, you may not know that the term for the right eye is oculus dexter, and the left eye is oculus sinister. Some eye doctors have modernized their systems, and are now using RE and LE to indicate right and left eyes, but most are still using the Latin. The right eye is always listed first in a prescription, because it will be on the optometrist’s left as they examine you, which is generally the “first” if you look at our left-to-right conventions for reading.
Indicates the amount of lens power you need. A positive number indicates that you are farsighted, and a negative number indicates you are nearsighted. Spherical magnifications are equal all the way around the eye, while aspherical magnifications are indicated with a cylinder notation, and correct for astigmatism (an unequal focal plane). The axis value is included when there is cylinder notation, and indicates the meridian (angle from the center of the eye) that has the most magnification when correcting for this condition.
Add indicates the magnification needed for the bottom portion of the lens when manufacturing multifocal lenses to correct presbyopia, the inability to see close-up most strongly associated with aging.